The present invention generally relates to communication control units, and more particularly to a communication control unit which is applicable to a multi-media communication typified by a local area network (LAN).
A multi-channel lattice communication network which is applicable to LANs, public line networks and particularly to a multi-media communication was previously proposed in a U.S. Pat. No. 4,839,887 in which the assignee is the same as the assignee of the present application. The proposed lattice communication network is analogous to nerve cells of a living body, and employs node apparatuses. Each node apparatus has a plurality of input and output channels and is used as a communication control element. Such node apparatuses are coupled in a multi-conjunction to form the lattice communication network. Each node apparatus transfers one of incoming digital signals according to a first-come-first-output logic.
The proposed lattice communication network has a large degree of freedom of network topology because of the multi-conjunction architecture. Thus, the survival rate of data is high. In other words, even when a fault occurs in a path of the communication network, it is possible to carry out the communication through other paths. Furthermore, it is possible to select optimum paths for the communication because each node apparatus transfers the incoming digital signals according to the first-come-first-output logic.
On the other hand, the proposed lattice communication network employs a multi-channel system in which a plurality of connecting channels are established, so as to effectively achieve a full duplex communication. The proposed lattice communication network is effectively applicable to the physical layer to the network layer of the open system interconnection (OSI), for example.
As other networks, there is the token ring network employing the ring topology of IEEE 802.5. A description of the algorithm of this token ring network will be omitted in this specification. The token ring network has superior security functions in that all terminals are synchronized, only the terminal which has a free token (that is, a token which indicates the right to transmit) is authorized to make a transmission, message information is returned to a transmitting terminal thereby making it possible to confirm whether or not the message information is correctly received by a receiving terminal and the like. Moreover, the token ring network has a satisfactory throughput delay characteristic when the traffic is high.
However, according to the token ring network, the transmitting terminal cannot start the transmission until the free token makes one round of the network, and the waiting time tends to become long. In addition, the performance of the token ring network deteriorates with system expansion. That is, in the token ring network, the time it takes for the free token to reach a terminal becomes longer as the number of terminals increases, and furthermore, the traffic becomes high at the same time, thereby making it even longer for the free token to reach a terminal. The token ring network also has a problem in that the survival rate of data is poor because the network is greatly affected by a fault in a center ring.
On the other hand, the proposed lattice communication network does not have the concept of address, and can be regarded as "a cable which has a predetermined algorithm". In other words, the proposed lattice communication network does not require a fixed protocol, and it is thus possible to connect terminals which use various protocols. Accordingly, when a system is formed by connecting the terminals of the token ring network to the lattice communication network, it is possible to anticipate elimination of the problems of the token ring network and realization of a token ring network which displays the advantageous features of the lattice communication network.
However, it is impossible to simply connect the terminals of the token ring network to the lattice communication network, and at least an interface unit is required to connect the token ring network and the lattice communication network. Such an interface unit and a token ring network adaptive unit (TAU) are further disclosed in a U.S. patent application Ser. No. 631,274 filed Dec. 20, 1990, the disclosure of which is hereby incorporated by reference.
On the other hand, the token ring network treats various kinds of frame signals. Hence, if a medium access control (MAC) frame which is used for ring management is transmitted from the terminal of the token ring network to the lattice communication network, the MAC frame may disappear in the lattice communication network due to a collision generated therein.
In addition, in the TAU described above, no special consideration is given as to the treatment of broadcast signals treated in the lattice communication network, and the broadcast signals are treated similarly as the frames having the usual individual addresses. As a result, the broadcast signal will not return to the transmitting terminal when the broadcast signal is transmitted, and the transmitting terminal in this case processes this situation as an abnormal state.
Furthermore, in the TAU described above, no retiming control can be made independently for the input frame from the lattice communication network and the input frame from the terminal. Moreover, a collision may occur between the two input frames.
In the TAU described above, the frame is transmitted to the lattice communication network on the assumption that the other end is active. Hence, even when the frame is transmitted to the lattice communication network, the frame does not necessarily return to the transmitting terminal if the power source of the terminal on the other end or the TAU is OFF. As a result, the transmitting terminal may see this non-return of the frame as a failure of the entire network such as a cable failure, and cannot appropriately cope with the situation.
On the other hand, when connecting the lattice communication network and the toke ring network, no measures are taken to avoid collision due to the detection of identical addresses and to transmit the carrier in advance. In other words, in the terminal of the token ring network, the collision which does not occur in the token ring network is generated by the connection to the lattice communication network. If the TAU detects the collision, the transmitting frame and the receiving frame are both cut off. As a result, when the collision is generated, the transmitting frame does not return to the transmitting terminal. Even if the transmitting frame does return to the transmitting terminal, the returned frame will not be complete in form and may be cut off at an intermediate part of the frame. Such a frame which is not complete in form may be input to the receiving terminal. Since such errors are not generated in the normal token ring network, the errors become treated as fatal errors and the terminal enters a complex process such as initializing the ring. Therefore, the communication efficiency becomes extremely poor.
In addition, if the destination address of the input frame from the lattice communication network is the address of its own station, this frame is transmitted from the TAU to the terminal and the frame is returned to the TAU from the terminal to be transmitted to the lattice communication network via the TAU. For this reason, the returning of the answer signal to the lattice communication network is delayed when the frame passes through the normal path, and it may be impossible to return the answer signal within the time in which the path is fixed.